This application claims the priority of Korean Patent Application No. 2003-35601, filed on Jun. 3, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Disclosure
The present disclosures relates to a semiconductor laser device, and more particularly, to a laser device having a smooth cleavage plane.
2. Description of the Related Art
A semiconductor laser is widely used to transfer, record, and read data in the field of communications, such as optical communications, or in devices, such as a compact disk player (CDP) and a digital versatile disk player (DVDP).
Since a semiconductor laser device may maintain oscillation characteristics of laser beam in a limited space, may be formed to a small scale, and requires a small critical current for laser oscillations, the semiconductor laser device is widely used. As the number of industrial fields to which the semiconductor laser is applied increases, demand for semiconductor laser devices having a smaller critical current increases. In other words, semiconductor laser devices having excellent characteristics, such as oscillating at a low current, and the ability to pass a lifespan test are needed.
In order to decrease the operating power and increase the output of the laser device, a smooth light-exiting surface, through which light exits the laser device, perpendicular to a laser oscillation layer is required. The light-exiting surface, which is formed by etching or scribing, is referred to as a facet or a cleavage plane.
When the light-exiting surface is formed by dry etching, the light-exiting surface is rough, resulting in a large optical loss and low reproducibility. However, when the cleavage plane is formed by scribing, the optical loss is reduced. A nitride semiconductor laser device, such as gallium nitride (GaN) uses the cleavage plane as the light-exiting surface. However, the crystal structures of GaN grown on a sapphire substrate and the sapphire substrate are different so that it is technically difficult to form a smooth cleavage plane and the yield is low.
FIG. 1 is a sectional view of a conventional nitride semiconductor laser device.
Referring to FIG. 1, an n-GaN lower contact layer 12, which is divided into a first region R1 and a second region R2, is stacked on a sapphire substrate 10. A multi-layered semiconductor material layer with a mesa structure exists on the lower contact layer 12. In other words, on the first region R1, an n-GaN/AlGaN lower cladding layer 24, an n-GaN lower wave guide layer 26, a InGaN active layer 28, a p-GaN upper wave guide layer 30, and a p-GaN/AlGaN upper cladding layer 32 are sequentially stacked on the n-GaN lower contact layer 12. The refractive indexes of the n-GaN/AlGaN lower cladding layer 24 and the p-GaN/AlGaN upper cladding layer 32 are smaller than the refractive indexes of the n-GaN lower wave guide layer 26 and the p-GaN upper wave guide layer 30. In addition, the refractive indexes of the n-GaN lower wave guide layer 26 and the p-GaN upper wave guide layer 30 are smaller than the refractive index of the active layer 28. In the mesa structure, a protruding ridge 32a having a predetermined width is formed at the center of the upper portion of the p-GaN/AlGN upper cladding layer 32, providing a ridge wave guide structure, and a p-GaN upper contact layer 34 is formed on the ridge 32a. A buried layer 36, which acts as a passivation layer having a contact hole 36a is formed on the p-GaN/AlGaN upper cladding layer 32. The contact hole 36a of the buried layer 36 is located over the upper contact layer 34 that is formed on the ridge 32a, and edge of the contact hole 36a overlaps the edge of the upper surface of the upper contact layer 34.
A p-type upper electrode 38 is formed on the buried layer 36. The p-type electrode 38 contacts the upper contact layer 34 through the contact hole 36a of the buried layer 36. In the second region R2, an n-type lower electrode 37 is formed on the lower contact layer 12, whose height is lower in the second region R2 than in the first region R1.
The ridge wave guide structure formed on the upper cladding layer 32 limits currents that are injected to the active layer 28 in order to limit a width of a resonance area for laser oscillation in the active layer 28. Thus, a transverse mode is stabilized and the operating current is lowered.
In the process of manufacturing the conventional nitride semiconductor laser device, the multi-layered GaN semiconductor material layer is formed on the sapphire substrate, and the ridge corresponding to a current injection area is formed by dry etching. Then, a mesa structure is formed on the n-GaN lower contact layer in order to expose the n-GaN lower contact layer and form the resonance surface. Such a mesa structure is formed as an array type on the sapphire substrate, and is then divided into unit devices by scribing. FIG. 2 is a plane view illustrating two mesa structures corresponding to two unit devices that are formed on the n-GaN contact layer 12. The mesa structures are interconnected by a connection unit 40 and share the ridge 32a, which crosses the connection unit 40. The mesa structures and the substrate, which supports the mesa structures, are divided into the unit devices along a scribing line A-A′ that intersects the connection unit 40.
As described above, the mesa structures are divided into the unit devices by scribing, and the cleavage planes from which a laser beam exits are formed at the edges resulting from the scribing. A GaN c-plane formed on a sapphire-c plane is tilted by about 30° toward the sapphire-c plane. Since the sapphire-c plane and the GaN c-plane are tilted, it is difficult to form a smooth cleavage plane perpendicular to the laser oscillation layer. In order to form the smooth cleavage planes perpendicular to the laser oscillation layer on the GaN semiconductor material layer, the cleavage plane of the sapphire substrate should be precisely divided by scribing. When the scribing force is transferred from the sapphire substrate to the lower portion of the mesa structure and the ridge at the upper portion of the mesa structure, the scribing force should not be concentrated at a specific location of the mesa structure, but should be evenly distributed.
The light-exiting surfaces, in other words, the cleavage planes, of the semiconductor material layer formed by the conventional method have little uniformity. In other words, the shapes of the cleavage planes are different from chip to chip even when the chips are manufactured under the same scribing conditions. The yield of laser devices proper for transmitting light, in other words, having the smooth cleavage plane perpendicular to the oscillation layer, is about 65%.
The following is an analysis of the laser device with the inferior light-exiting surface. When scribing the mesa structure by transferring the scribing force from the sapphire substrate to the mesa structure, the scribing force is concentrated at a lower corner of the mesa structure so that cracks occur at the lower corner of the mesa structure as shown in the dotted rectangle of FIG. 3. Here, the cracks are transferred to the light-exiting surface. Another inferior light-exiting surface is caused by cracks in a GaN coalescence formed by epitaxial lateral overgrowth (ELOG), which is disclosed in U.S. Pat. No. 6,348,108. Referring to FIG. 4, when the scribing force is transferred from the sapphire substrate to the GaN, cracks occur at the GaN coalescence. The cracks are transferred to a ridge wave guide formed on the mesa structure as shown in the dotted rectangle of FIG. 4, so that a rough cleavage plane is formed.
The cracks and the rough cleavage plane result in a decrease in optical output and an increase in operating current.